Hi. I could use a little guidance-
I'm attempting to fix a friend's Mono/Poly. A vintage unit - pc board version indicated are KLM354C if that helps. I"m using this schematic:
http://dl.lojinx.com/analoghell/KorgMonoPoly-ServiceManual.pdf
It has no output. Supply voltages seem to be present. I've managed to close the trigger in switch which turned on the VCO LEDs and gave me a 4vpp square wave on the output of the FETs 401,301,201,101. (There is no signal there without closing that switch.) The keys have no effect on the signal at this point, and I'm not seeing signal on any of the legs of the 4 VCO level pots VR11-14, or anywhere downstream.
It's possible I may be complicating matters by having the external controls set up wrong, or some other operational mistake. The owner says it worked, and he opened it up to try to clean some keys but got cold feet. He was surprised that we couldn't get anything out of it - he brought it to me just to fix some key contacts. All the connectors are solidly seated.
Would someone like to nudge me in the right direction?I haven't done a keyboard repair in quite a few years.

It would seem reasonable to start looking at the key assigner section first, to check that the keyboard keypresses are being recognised.

Bear in mind that the circuitry is likely to be quite sensitive to static-induced damage, so take precautions when working on it. Also try to establish whether there was a chance it might have been zapped when opened by the owner.

The key assigner basically runs in code on a uPD8049 8-bit microcomputer. The keyboard lines are shared with other functions, such as controlling the LEDs. One way of checking that the computer is running is to check the response of the Poly, Unison, Chord, and Hold LEDs in response to switch changes.

Is it possible that the cable harness between keyboard and the corresponding PCB assembly (KLM356?) has been connected the wrong way round? The drawings suggest the connectors on the PCB should prevent incorrect re-assembly, but it's worth checking, all the same, both at the keyboard PCB end and at the KLM356 part.

Can you point out a couple points on the KLM-356 where I could see some action when the keys are pressed? Seems like the next thing to do.
I don't know what the 4 VCO LEDs actually indicate, but I have shorted across the Trig jack to turn them all on. Seems right. On IC3, pins 1,2,10, and 11 have 0v whether keys are pressed or not.
With the scope, I've poked around IC2 and IC6 and those 4069 outputs, and I'm not seeing any response to the keyboard. some 5V, and some clock signals, no keyboard response.

The multiplexed nature of the synth circuitry significantly reduces the actual circuit complexity and the amount of wiring, but makes fault tracing a bit of a nightmare.

That the 5 status LEDs light in response to their respective switch positions, tells us several things. 1) that IC2 seems to be OK, 2) 5 of those 6 data lines from the microprocessor to IC2 work as they should (pin 4 of IC2 is shown as not connected to anything in the schematic). Those data lines are also tied to some of the keyboard lines (K1 to K6), and four of them also go to IC3. 3) The data lines connected to those switches must be working. Those data lines are also shared by some of the keyboard lines (K7, K8, K9, K10 and K12). If the keys sharing those lines are pressed then, provided there is continuity in the keyboard circuitry (i.e. wiring, switch contacts and diode matrix are all OK), then the microprocessor code should be able to make sense of which keys have been pressed, provided the processer code is not broken. First, though, it's worth looking at what is happening with IC3. The data lines to it are OK, based on the above. Check what IC3 clock line (pin 5) is doing, using an oscilloscope. It should briefly pulse high every time a keyboard key is pressed, and probably again when it is released, though this may not correspond to the moment the key has been released. This should latch the state of the data lines onto IC3's outputs, which are the lines that send the gate signals out via IC1. If the IC3 clock pin does pulse in response to keyboard presses, that's encouraging. At least one of the output pins (1,2,10,11) should go high, and then low again on key release. If there is a clock pulse, but none of the outputs go high, then replace IC3.

If IC3 clock pin does go high, then also look at the output on pin 1 of IC8. The voltage on this pin should vary, with the level depending on the keyboard key being pressed at the time. If it does vary sensibly, then move on to the next bit of the circuit. The schematic is a bit indistinct here, but the signal should find its way, suitably scaled and modified, to pin 3 of IC6. If it doesn't, try to establish why. Is the switch in the CV jack making good contact, for example. If there are CV signals on pin 3 of IC6, then look for the same signal level on one of the output pins of IC4 or IC5. The output pins of IC6 form keyboard-generated CV signal sample and hold circuits with C1, C2, C3 and C16. Trying to measure the signals here with anything other than a very high impedance voltmeter is likely to give readings of 0V (the time constant of the above capacitors with a 1 Meg scope probe is 47msec). IC4 and IC5 are buffer amplifiers for the CV sample and hold.

Maybe something is not right with the clock signal coming into IC3. First of all, I am seeing no response to the keys.
The signal I'm seeing is odd, at pin 5 of IC3. this may involve my fluke 97 50MHz scope. Early digital, LCD. I inherited it somewhere along the way, and I've used it a lot but only in analog circuits. bear with me.
At 5microsec/div, there is one +5 pulse about 5us wide, and the remainder of the display is 0V. If I change the div time, the one pulse is larger or smaller, but no further pulses appear. Perhaps this is anomalous, or I'm having a hard time capturing a pulse. Do you have any guess as to how wide the pulse should be?
Beyond that, as you said, the data busses coming into IC3 are fine yet there is no voltage change on any of the gate signal. So, the keyboard may not be putting out anything, the proc may not be sending signal to IC3, or IC3 is locking up that clock line. Since the data busses work I guess that means that there isn't another part of the circuit holding down IC13, like pin 4 or 6.
IC12 also gets this same clock. Those outputs are 0v as well, no key response. What do you think I should do next? Is there anything else to ascertain here before swapping ICs? Maybe both IC3 and IC12 before worrying about the proc? It is the only thing socketed - would you pull it and try to get IC3 to function?

The pulse at IC3 clock pin will be brief, probably similar to that appearing on IC2's clock pin when one of the switches is pressed. Expect a single brief pulse on IC2 pin 9 when one of the switches is pressed. 5us seems about the right ball park. It only requires a very brief pulse to latch the input pin state onto the outputs of IC2 and IC3.

What happens at IC8 pin1 when keyboard keys are pressed? This is the output of the op-amp that buffers the CV signal from the keyboard. The voltages here should change as the keys are pressed, i.e. not gated, increasing as the notes are played higher up the scale.

Thanks so much for the help.
I'm getting the same scope display on IC2 pin 9 - 5us/div, DC, and there is one stationary, constant +5v about 5us wide, and no response when I press the mode buttons. Which are still working.
Tried a lot of settings, but I can't get my scope to tell me anything about these pulses.
IC8 pin 1 does not react to keys. It sits at 5V.

By stationary constant +5V on IC2 pin 9, does this mean there are regularly repeated pulses on this pin? If so, then it seems that the multiplexing runs to a regular schedule, rather than being influenced by "demand" from the various switch and keyboard sources. If the clock pulses to IC2 and IC3, were to be looked at using two channels of the scope, the pulses would be separated in time.

The fact that IC8 pin 1 does not react to key presses needs to be investigated, as it seems the keyboard keypresses aren't being recognised. Either work back from IC8 pin 1, or look at the keyboard itself, and the associated circuitry, wiring and connectors.

The keyboard schematic can be used as a basis for checking that it works as it should by disconnecting the keyboard plug and starting with pin 1 and pin 10 check with an ohmmeter that contact is made when the topmost key is pressed. The ohmmeter +ve should be to pin 10, to ensure that the corresponding diode conducts. Work through the matrix with pins 7 to 14 connected to positive, and pins 1 to 6 to negative.

Now I am testing continuity on the keyboard. I am getting continuity from connector pins 7-14 to the various anodes, but I couldn't find any of the cathodes while pressing down handfuls of keys. I'm testing from pins 1-6 to any of the cathodes, and pressing each key. I cannot get any continuity. Not one. That seems crazy.
I can't really make out the specific note designations in the drawing so I'm testing every key while my meter is connected to any of the 1-6 pins and any selected cathode. How can this be? is there something about the mechanical assembly that can sever the connection to pins 1-6? Is every single key contact dirty enough to not work at all?

Hmmmm, strange indeed, but this could turn out to be the problem. Is there any chance the contact assembly itself is somehow not working as it should, or a crack or deep scratch to the PCB? Is there continuity between pins 1 - 6 and their respective groups of contacts? The key labelling on the keyboard schematic is indistinct, but it starts from the left hand side with F1, F#1, G1, G#1, A1.... and ends with ....A4, A#4, B4, C5, so bsically going up the keyboard, key by key from left to right.